PROJECT SUMMARY Flow cytometry is a workhorse technique in research and development as well as in clinical laboratories for diagnosis and monitoring of disease. It is particularly useful in distinguishing between populations of immune cells based on their expressed cell surface antigens. Standard flow cytometers use fluorescent tags, often conjugated to monoclonal antibodies, to give qualitative and quantitative information about specific molecules in the cell. This molecular specificity, coupled with the fact that information is obtained on a cell-by-cell basis with very high throughput (up to 30,000 cells per second), make this a powerful technique. The ability to multiplex (measure a variety of different molecular species in a single cell) further adds to its utility and to the complexity of the scientific questions that can be addressed using this technique. However, the level of multiplexing currently has limitations. Flow cytometry analysis typically relies solely on spectral information of the fluorescent tags and is thus limited by the spectral overlap of fluorophore emissions. Currently, employing even moderate levels of multiplexing requires complex instrumentation and careful experimental design, execution and analysis to compensate for spectral spillover of signal into multiple channels. This severely limits the range of scientific questions that can be addressed using current technologies, deters novices in the technique from attempting more complex yet scientifically relevant experiments, and is widely regarded as the major bottleneck in the field. To overcome this limitation, we propose to build on our results from Phase I where we demonstrated feasibility for an innovative approach that uses fluorescence lifetime as a separate, additional discriminating measurement parameter. Our scheme for using fluorescent lifetime for multiplexing is simple, scalable, and supported by preliminary data from our prototype instrument. Here we propose to upgrade our Phase I instrument to increase multiplexing capability, challenge that instrument with a battery of verification tests, and validate using a relevant biological assay, benchmarking results against a conventional flow cytometer. The result will be a system enabling compensation-free flow cytometry experiments of 12 colors, while requiring fewer lasers and detectors than similarly equipped commercial systems. Such a system would serve a large segment of the market, including clinical cytometry, and is expected to see broad adoption. This would pave the way for further development of an ultra-high (30+) parameter instrument suitable for immunophenotyping, yet requiring significantly less compensation than current systems, and pushing the boundaries of experimental complexity. Given flow cytometry?s wide-spread use and importance, this project will have a high impact in many biomedical and clinical applications. Several large instrumentation companies have already indicated an eagerness to engage in strategic partnerships aimed at commercialization.